This right here is why you are my hero. I've been wanting ZFS since it came out but was both waiting for the code to mature and for prices to come down enough enough to make my raid5 array long obsolete. Who would have thunk it, me actually wanting/preferring software raid but the features are too hard to resist. 1.8 * 7 = Just over 12 TB, enough to make a grown geek cry.

I have to admit I only just skimmed over the whole thing the first time but I'm going to have to go back and reread it a few times. Still not sure why you created the sparse files/md drives. I though ZFS had a simple add to pool command.

It is not possible to add a disk as a column to a RAID-Z, RAID-Z2, or RAID-Z3 vdev. This feature depends on the block pointer rewrite functionality due to be added soon. You can however create a new RAID-Z vdev and add it to the zpool.

I should explain this in more detail. In short, I had to create a 7 vdev array with only 5 physical HDD.